Bottom Line:
It was found to represent not only a new genus, Tranatocetus gen. nov., but also a new family; Tranatocetidae.Inclusion of problematic taxa like Tranatocetus argillarius in phylogenies brings new understanding of the distribution and diagnostic value of character traits.This underlines the need for re-examination of earlier described specimens in the light of the wealth of new information published in later years.

ABSTRACTMiocene baleen whales were highly diverse and included tens of genera. However, their taxonomy and phylogeny, as well as relationships with living whales, are still a subject of controversy. Here, "Mesocetus" argillarius, a poorly known specimen from Denmark, is redescribed with a focus on the cranial anatomy. It was found to represent not only a new genus, Tranatocetus gen. nov., but also a new family; Tranatocetidae. The whales of this family have the rostral bones either overriding or dividing the frontals; the rostral bones are contacting the parietals and nasals dividing the maxillae on the vertex; the occipital shield is dorsoventrally bent. The tympanic bulla is particularly characteristic of this family featuring a short, narrow anterior portion with a rounded or squared anterior end and a wider and higher posterior portion that is swollen in the posteroventral area. A phylogenetic analysis including 51 taxa supports a monophyletic group comprising most Neogene and modern whales, with Tranatocetidae being possibly closer related to Balaenopteridae (rorquals) than to Cetotheriidae. Tranatocetidae exhibit a charahteristic bulla shape. In fact, all Neogene and modern mysticete families examined have a unique shape of the tympanic bulla that is diagnostic at family-level. Inclusion of problematic taxa like Tranatocetus argillarius in phylogenies brings new understanding of the distribution and diagnostic value of character traits. This underlines the need for re-examination of earlier described specimens in the light of the wealth of new information published in later years.

pone.0135500.g010: The phylogenetic tree of Tranatocetus argillarius and related taxa of baleen whales.The tree is the consensus of 12 most parsimonious trees (501 step, CI = 0.33, RI = 0.66). The age values are provisionally indicated as the earliest estimates and are based on the review by Fordyce and Marx [8], as well as on original descriptions.

Mentions:
The phylogenetic analysis supports a monophyletic Plicogulae comprising Tranatocetidae, Balaenopteridae, Eschrichtiidae, Cetotheriidae and Neobalaenidae, as well as a number of mid-Miocene whales (Fig 10). Plicogulae split into two major branches, one containing Cetotheriidae and Neobalaenidae, the other Tranatocetidae, Balaenopteridae and Eschrichtiidae. The basalmost described cetotheriid is Joumocetus shimizui [20]. The clade including the recent pygmy right whales (Neobalaenidae) also includes the Late Miocene genus Cephalotropis and has Pinocetus polonicus from the Middle Miocene of Poland [21] as the basalmost member. The stem taxa for both Cetotheriidae and Neobalaenidae are the Middle Miocene taxa Otradnocetus and Parietobalaena, the latter of which appears as a paraphyletic group. Thus, Cetotheriidae and Neobalaenidae, as well as Cephalotropis, Pinocetus, Otradnocetus and Parietobalaena, are classified here as superfamily Cetotherioidea [5]. Other mid-Miocene non-balaenid taxa included in this analysis are classified as stem Plicogulae (Diorocetidae) or stem members of the Balaenopteridae clade. Balaenopteridae and Eschrichtiidae form a clade with Titanocetus [22] and Uranocetus [23]. Tranatocetidae is the sister group to this clade. Tranatocetus argillarius and Mesocetus longirostris [24, 25] are pooled together in the cladogram based on the common primitive traits: anteroventrally directed glenoid fossa, sigmoidal nuchal crests and the large mandibular foramen. Tranatocetidae also includes “Aulocetus” latus, “Cetotherium” megalophysum, “Cetotherium” (or “Metopocetus”) vandelli and Mixocetus elysius. Another related taxon, not included in the analysis, is “Plesiocetopsis hupschii”, as identified by Van Beneden [25] (Fig 7E–7H) which is represented by the isolated fragments: it is unclear if it relates to the holotype of this species [26, 27] which is currently unavailable for study [28]; in its turn, this holotype, as illustrated by [27], is strongly similar to Mesocetus longirostris [25]. All of these taxa have been mentioned and described as representatives of different families and genera [5, 7, 10, 28–33], and their anatomy, taxonomy and relationships need further revision.

pone.0135500.g010: The phylogenetic tree of Tranatocetus argillarius and related taxa of baleen whales.The tree is the consensus of 12 most parsimonious trees (501 step, CI = 0.33, RI = 0.66). The age values are provisionally indicated as the earliest estimates and are based on the review by Fordyce and Marx [8], as well as on original descriptions.

Mentions:
The phylogenetic analysis supports a monophyletic Plicogulae comprising Tranatocetidae, Balaenopteridae, Eschrichtiidae, Cetotheriidae and Neobalaenidae, as well as a number of mid-Miocene whales (Fig 10). Plicogulae split into two major branches, one containing Cetotheriidae and Neobalaenidae, the other Tranatocetidae, Balaenopteridae and Eschrichtiidae. The basalmost described cetotheriid is Joumocetus shimizui [20]. The clade including the recent pygmy right whales (Neobalaenidae) also includes the Late Miocene genus Cephalotropis and has Pinocetus polonicus from the Middle Miocene of Poland [21] as the basalmost member. The stem taxa for both Cetotheriidae and Neobalaenidae are the Middle Miocene taxa Otradnocetus and Parietobalaena, the latter of which appears as a paraphyletic group. Thus, Cetotheriidae and Neobalaenidae, as well as Cephalotropis, Pinocetus, Otradnocetus and Parietobalaena, are classified here as superfamily Cetotherioidea [5]. Other mid-Miocene non-balaenid taxa included in this analysis are classified as stem Plicogulae (Diorocetidae) or stem members of the Balaenopteridae clade. Balaenopteridae and Eschrichtiidae form a clade with Titanocetus [22] and Uranocetus [23]. Tranatocetidae is the sister group to this clade. Tranatocetus argillarius and Mesocetus longirostris [24, 25] are pooled together in the cladogram based on the common primitive traits: anteroventrally directed glenoid fossa, sigmoidal nuchal crests and the large mandibular foramen. Tranatocetidae also includes “Aulocetus” latus, “Cetotherium” megalophysum, “Cetotherium” (or “Metopocetus”) vandelli and Mixocetus elysius. Another related taxon, not included in the analysis, is “Plesiocetopsis hupschii”, as identified by Van Beneden [25] (Fig 7E–7H) which is represented by the isolated fragments: it is unclear if it relates to the holotype of this species [26, 27] which is currently unavailable for study [28]; in its turn, this holotype, as illustrated by [27], is strongly similar to Mesocetus longirostris [25]. All of these taxa have been mentioned and described as representatives of different families and genera [5, 7, 10, 28–33], and their anatomy, taxonomy and relationships need further revision.

Bottom Line:
It was found to represent not only a new genus, Tranatocetus gen. nov., but also a new family; Tranatocetidae.Inclusion of problematic taxa like Tranatocetus argillarius in phylogenies brings new understanding of the distribution and diagnostic value of character traits.This underlines the need for re-examination of earlier described specimens in the light of the wealth of new information published in later years.

ABSTRACTMiocene baleen whales were highly diverse and included tens of genera. However, their taxonomy and phylogeny, as well as relationships with living whales, are still a subject of controversy. Here, "Mesocetus" argillarius, a poorly known specimen from Denmark, is redescribed with a focus on the cranial anatomy. It was found to represent not only a new genus, Tranatocetus gen. nov., but also a new family; Tranatocetidae. The whales of this family have the rostral bones either overriding or dividing the frontals; the rostral bones are contacting the parietals and nasals dividing the maxillae on the vertex; the occipital shield is dorsoventrally bent. The tympanic bulla is particularly characteristic of this family featuring a short, narrow anterior portion with a rounded or squared anterior end and a wider and higher posterior portion that is swollen in the posteroventral area. A phylogenetic analysis including 51 taxa supports a monophyletic group comprising most Neogene and modern whales, with Tranatocetidae being possibly closer related to Balaenopteridae (rorquals) than to Cetotheriidae. Tranatocetidae exhibit a charahteristic bulla shape. In fact, all Neogene and modern mysticete families examined have a unique shape of the tympanic bulla that is diagnostic at family-level. Inclusion of problematic taxa like Tranatocetus argillarius in phylogenies brings new understanding of the distribution and diagnostic value of character traits. This underlines the need for re-examination of earlier described specimens in the light of the wealth of new information published in later years.